This excellent review confirms the previous meta-analysis...
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This excellent review confirms the previous meta-analysis by Henquet et al. (2005) and as such does not add anything new. The importance lies in the UK context: previously the Lancet has been mostly skeptical with regard to this issue. The fact that the leading UK medical journal now also allows these findings to see daylight is a significant event and helps stimulate further funding for the effort that several groups worldwide have started working on over the last five years: the search for the mechanism explaining the link.

It is reassuring to see that the results of the latest meta-analysis (Moore et al., 2007) are consistent with previous meta-analyses, and that the various meta-analyses are broadly consistent with the now much-tortured primary data. Despite the meta-analysis fatigue, the results are too important to ignore.

When thinking about the impact of cannabis on schizophrenia frequency measures, it is important to remember that cannabis use may translate to an increase in the prevalence of active psychosis via two mechanisms. The data suggest that as the prevalence of cannabis use increases in a population, the incidence of schizophrenia should also increase (Hickman et al., 2007). Furthermore, in those with established schizophrenia, cannabis use is associated with poorer outcomes (i.e., reduced remission rates). Thus, from a modeling perspective, increased cannabis use could lead to an increase in the prevalence of active psychosis via two mechanisms (i.e., increased “inflow” and decreased “outflow”) (McGrath and Saha, 2007).

The prevalence of active psychosis in the community may be “under the influence” of cannabis from more than one perspective.

The recent meta-analysis in the Lancet (Moore et al., 2007) regarding cannabis use and psychotic or affective mental health outcomes is, indeed, a necessary contribution. It is the first systematic review restricted to longitudinal studies of cannabis use and mental health outcomes. For this addition to the contours of the literature, Zammit and colleagues are to be commended.

We may be more optimistic than the authors, however, about the potential for future longitudinal studies to shed further light on the question of causality, and perhaps more cautious about the present state of the evidence. Given the public health and policy implications, we propose a concerted effort to complete observational studies that are designed to rule out the main alternative explanations for the association (e.g., genetic or social factors that independently influence both cannabis use and psychosis). The Swedish conscript study (Zammit et al., 2002) is a fine example of one such study. We should also be considering natural experiments and designs based on instrumental variables enabled by in order to complement this work. For instance, we might capitalize on situations created by policy changes that affect the availability—and therefore use—of cannabis in order to examine the impact on the development of psychosis. Whether at the individual or the population level, both creativity and rigor are required.

Current interest in cannabis and the onset of psychosis is...
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Current interest in cannabis and the onset of psychosis is laudable. The Lancet paper no doubt establishes a causal link based upon what has been known in the literature (Raphael et al., 2005; Roberts et al., 2007; Rey et al., 2004; Wittchen et al., 2007). The authors need to be congratulated for taking extreme care to incorporate most of the studies and also for making conclusions with a sense of skepticism. That is where further questions arise.

1. Cannabis is used only in certain cultures and known to be involved in a maximum 50 percent of cases of psychosis, schizophrenia, and schizophreniform psychosis (Gregg et al., 2007). In that sense, are there two different phenotypes of schizophrenia, a) where exposure to cannabis is necessarily a factor and b) where a different set of potentiating or precipitating factors work, not cannabis?

2. Even if we focus only on the first possibility, there are few unanswered questions such as, what are the concurrent clinical conditions along with cannabis abuse? Do these patients have cognitive dysfunction? Is that reflective of broader brain mechanism changes?

3. There seems to be no reliable biological explanation as to why exposure to cannabis should precipitate psychosis. Cannabis is one of the most commonly used illicit drugs. Its active compound “cannabidols” has 64 active isomers, each having differing effects on health and behavior. There is strong support for a link between cannabis and development—exacerbation of psychosis as well as other mental health conditions (e.g., anxiety, depression). Further research is needed to determine the underlying neurochemical processes and their possible contributions to etiology, as well as the social factors that contribute to the increasing use of cannabis by young people.

4. There is a theory that preexisting cognitive dysfunction is a core feature of schizophrenia. Accepting this, there are no studies to show “causal relationship” between cannabis and cognitive dysfunction.

The current levels of information and understanding, though collected over last 25-30 years of research, are far from adequate to establish any direct relationship except “mere association.” It is hoped that more precise biological, imaging, and neuropsychological studies would be able to throw fresh light on this important area of research.

Acute cannabis administration can induce memory impairments, sometimes persisting months following abstinence. There is no evidence that residual effects on cognition remain after years of abstinence. The scarce literature on neuroimaging, mainly done in non-psychotic populations, shows little evidence that cannabis has effects on brain anatomy. Acute effects of cannabis include increases of cerebral blood flow, whereas long-term effects of cannabis include attenuation of cerebral blood flow. In animals Δ9-tetrahydrocannabinol enhances dopaminergic neurotransmission in brain regions known to be implicated in psychosis. Studies in humans show that genetic vulnerability may add to increased risk of developing psychosis and cognitive impairments following cannabis consumption. Δ9-tetrahydrocannabinol induces psychotic-like states and memory impairments in healthy volunteers (Linszen et al., 2007).

On the basis of six studies, it is concluded that there was insufficient evidence to prove conclusively that long-term cannabis use causes or does not cause residual abnormalities. The results of several reviews were also inconclusive as to whether cannabis use during adolescence may have a lasting effect on cognitive functioning and brain structure. However, it could not rule out that a) certain cognitive and cerebral abnormalities existed in patients before cannabis use began and b) that patients were suffering from subacute effects of cannabis (Weeda et al., 2006).

Continued cannabis use by persons with schizophrenia predicts a small increase in psychotic symptom severity but not vice versa (Degenhardt et al., 2007). Currently, there is a lot of interest in cannabis use as a risk factor for the development of schizophrenia. Cognitive dysfunction associated with long-term or heavy cannabis use is similar in many respects to the cognitive endophenotypes that have been proposed as vulnerability markers of schizophrenia. In this situation, we need to examine the similarities between these in the context of the neurobiology underlying cognitive dysfunction, particularly implicating the endogenous cannabinoid system, which plays a significant role in attention, learning, and memory, and in general, inhibitory regulatory mechanisms in the brain. Closer examination of the cognitive deficits associated with specific parameters of cannabis use and interactions with neurodevelopmental stages and neural substrates will better inform our understanding of the nature of the association between cannabis use and psychosis. The theoretical and clinical significance of further research in this field is enhancing our understanding of underlying pathophysiology and improving the provision of treatments for substance use and mental illness (Solowij et al., 2007). Many studies now show a robust and consistent association between cannabis consumption and the ulterior development of psychosis. Furthermore, our better understanding of cannabis biology allows the proposal of a plausible hypothetical model, based notably on possible interactions between cannabis and dopaminergic neurotransmission (Jockers-Scherubl, 2006). Do they suffer from other disorders, which are underlying or may be causal or comorbid, and do these comorbid conditions also have neurocognitive changes, e.g., psychosis, ADHD, LD, Tourette disorder, and other movement disorders, or depression? Is there an interrelationship among these factors to cause abuse and degree of cannabis consumption?